STM32F103C8 - 8Mhz 33pf xtal ?

[pilot]

The datasheet suggests load of 20pF .

Due to size constraints, I hope to use https://jlcpcb.com/partdetail/318914-CSTNE8M00G550000R0/C341522

Most 8Mhz resonators of this size seems to have 33pF

[STOne-32]

Dear @pilot ,

Resonators are a bit different from Crystals as they have their built-in CL1 and CL2, this murata resonator has 33pF C1/C2 which is different from Crystal CL . In general CL1/CL2 value are in range of twice for crystal for the value of CL  - Stray /Parasitic .

Here this resonator is fully compatible with STM32F1 series . You can use no worries.

 

You can also refer to our Application Note :

https://www.st.com/resource/en/application_note/an2867-guidelines-for-oscillator-design-on-stm8afals-and-stm32-mcusmpus-stmicroelectronics.pdf#

I’m interesting to know the end application usage, this resonator has 8MHz ( -/+ 0.5%) you can use built-in HSI . 

Hope it helps ,

STOne-32.

[liaifat85 · Accepted solution]

Using a resonator with 33pF instead of the recommended 20pF might slightly shift the resonant frequency and increase startup time. This effect is usually small but may vary based on the sensitivity of your application.

[pilot]

Thank you, will test. Startup time is not an issue, nor is the frequency precision very important.

[pilot]

@STOne-32  The application consists basically of the device doing AD  , communicating on I2C  (100khz) and serial(USB)  I have simply no low frequency oscillator as there is no clock/sleep.

I must say I did not even look at the option to use HSI.

The device will see temperatures within -20+40°C 

[Tesla DeLorean]

For USB you want HSE

Also HSI+PLL gets you to 64 MHz on the F1

[pilot]

Ah, thank you - I knew about 64Mhz, but that is plenty enough, but the USB was a good point 

[STOne-32]

For USB : System Clock should be either 72Mhz or 48MHz . A 8MHz crystal with 0,05% precision is a must . I’m not sure that the precision of the selected resonator is safe .